1. Technical Field
The present invention relates to an optimized method for performing an electrolysis process for producing aluminium in accordance with the Hall-Héroult process with prebaked anodes, and anodes therefore.
2. Description of the Related Art
In a process as described above, there will be evolved gas at the wear-surface (primary the underside or bottom side) of anodes due to the reduction of alumina. In particular carbon dioxide gas will accumulate at this surface, causing variations and instabilities in the electrical contact from the anode to the electrolyte. This physical phenomena have several drawbacks, such as:                Increased back reaction and loss of current efficiency due to close contact between the produced aluminium layer and CO2 gas bubbles.        Increased possibility and duration of anode effects        Heat production in the gas layer results in a reduced interpolar distance and reduced current density on the cell. An increase in current density will increase the production on the cells.        
The extra IR-drop (Interpolar Resistance drop) because of the gas bubbles in the electrolyte has been measured to be 0.15-0.35 volt in alumina reduction cells (1992, The 11th International Course on Process Metallurgy of Aluminium page 6-11).
There have been several proposals for minimizing the above mentioned problem, such as introducing anodes with a sloped or tilted bottom, forming slots or tracks in the wear surface of the anodes to drain the gas away from this area.
Slots in prebaked anodes are normally produced in a vibrator compactor when the anode mass is in a green state, or in a dry milling process that is performed on the calcinated anodes. The dry milling process is normally performed by the use of a circular saw. In accordance with commonly available production methods of today, slots can be produced with a width that is approximately 13-15 mm.
There are some minuses by having slots in the anode surface, and it will be mentioned here:                Reduced anode life time in the cell because anode mass is removed        Reduced anode working surface area        Extra carbon material have to be transported back to the carbon mass factory (dry milling)        Extra energy In the milling operation (dry milling)        
All these drawbacks can be reduced by making the slots more narrow. Thus, the slots should not be wider than necessary to effectively drain the anode gases from the working surface
A study carried out and reported in “R. Shekar, J. W. Evans. Physical modelling studies of electrolyte flow due to gas evolution and some aspects of bubble behaviour in advanced Hall cells, Part III. Predicting the performance of advanced Hall cells, Met. and Mat. Trans.,Vol. 27 B. February 1996, pp. 19-27”, indicates that tracks with a width less than 1 cm did not drain the gas properly.